Rename fail! to panic!

https://github.com/rust-lang/rfcs/pull/221

The current terminology of "task failure" often causes problems when
writing or speaking about code. You often want to talk about the
possibility of an operation that returns a Result "failing", but cannot
because of the ambiguity with task failure. Instead, you have to speak
of "the failing case" or "when the operation does not succeed" or other
circumlocutions.

Likewise, we use a "Failure" header in rustdoc to describe when
operations may fail the task, but it would often be helpful to separate
out a section describing the "Err-producing" case.

We have been steadily moving away from task failure and toward Result as
an error-handling mechanism, so we should optimize our terminology
accordingly: Result-producing functions should be easy to describe.

To update your code, rename any call to `fail!` to `panic!` instead.
Assuming you have not created your own macro named `panic!`, this
will work on UNIX based systems:

    grep -lZR 'fail!' . | xargs -0 -l sed -i -e 's/fail!/panic!/g'

You can of course also do this by hand.

[breaking-change]

1 files changed, 5 insertions, 6 deletions

diff --git a/src/libstd/io/buffered.rs b/src/libstd/io/buffered.rs
index 95c44e6a3fc..9cd8dbcc509 100644
--- a/src/libstd/io/buffered.rs
+++ b/src/libstd/io/buffered.rs
@@ -183,7 +183,7 @@ impl<W: Writer> BufferedWriter<W> {
     ///
     /// The buffer is flushed before returning the writer.
     pub fn unwrap(mut self) -> W {
-        // FIXME(#12628): is failing the right thing to do if flushing fails?
+        // FIXME(#12628): is panicking the right thing to do if flushing panicks?
         self.flush_buf().unwrap();
         self.inner.take().unwrap()
     }
@@ -214,7 +214,7 @@ impl<W: Writer> Writer for BufferedWriter<W> {
 impl<W: Writer> Drop for BufferedWriter<W> {
     fn drop(&mut self) {
         if self.inner.is_some() {
-            // dtors should not fail, so we ignore a failed flush
+            // dtors should not panic, so we ignore a panicked flush
             let _ = self.flush_buf();
         }
     }
@@ -612,7 +612,7 @@ mod test {
 
     #[test]
     #[should_fail]
-    fn dont_fail_in_drop_on_failed_flush() {
+    fn dont_panic_in_drop_on_panicked_flush() {
         struct FailFlushWriter;
 
         impl Writer for FailFlushWriter {
@@ -623,9 +623,8 @@ mod test {
         let writer = FailFlushWriter;
         let _writer = BufferedWriter::new(writer);
 
-        // Trigger failure. If writer fails *again* due to the flush
-        // error then the process will abort.
-        fail!();
+        // If writer panics *again* due to the flush error then the process will abort.
+        panic!();
     }
 
     #[bench]